1. Technical Field
The present invention relates to semiconductor devices including light-emitting diodes, electronic devices, and semiconductor sensors, and to methods of manufacturing the semiconductor devices; in particular the present invention relates to semiconductor devices incorporating a reduced-dislocation-density GaN substrate, and to methods of manufacturing such devices.
2. Description of the Related Art
Designing for improved characteristics in various GaN-substrate or other III-nitride-substrate employing semiconductor devices, such as light-emitting diodes, electronic devices, and semiconductor sensors, is demanding low dislocation density from the substrates.
Examples that have been proposed of how to manufacture such III-nitride substrates of low dislocation density include the following. X. Xu et al., in “Growth and Characterization of Low Defect GAN by Hydride Vapor Phase Epitaxy,” Journal of Crystal Growth, 246, (2002), pp. 223-229 (“Non-Patent Literature 1” hereinafter) report that dislocation density decreases with increasing thickness of the grown crystal, and that, for example, growing GaN crystal to a thickness of 1 mm or more on a normative substrate of chemical composition different from that of GaN lowers the dislocation density to a level of 1×106 cm−2 or less.
Meanwhile, A. Usui et al., “Thick GaN Epitaxial Growth with Low Dislocation Density by Hydride Vapor Phase Epitaxy,” Japanese Journal of Applied Physics, Vol. 36 (1997), pp. L899-L902 (“Non-Patent Literature 2” hereinafter) report that in growing GaN crystal onto a non-native substrate, creating facets by forming a mask layer having apertures makes it possible to control the orientation in which dislocations propagate, and thereby lower the GaN crystal dislocation density.
Nevertheless, GaN crystal, and GaN substrates obtained from the crystal, grown by the crystal growing methods of Non-Patent Literature 1 or Non-Patent Literature 2, proved to be plagued with serious defects apart from dislocations, although the density of the dislocations is in fact lowered to about 1×106 cm−2. The defects were readily detected, inasmuch as the GaN substrate was etched with an alkali, leaving it pitted. In particular, when a specular-polished (0001) Ga face of a GaN substrate was etched for some tens of minutes in an aqueous KOH solution at 50° C., the areas where defects were present were etched to a depth of several μm, forming pits. Furthermore, etching the specular (0001) Ga face of the GaN substrate with molten KOH, molten NaOH melt, or a molten KOH/NaOH mixture, pitted the Ga face with roughly hexagonal columns, walled by N faces.
GaN is a crystal having polarity in the [0001] direction, and a characteristic trait of GaN crystal is that its (0001) Ga faces are not readily etched with alkalis, whereas its (000 1) N faces are readily alkali-etched. From this perspective, it is apparent that the GaN crystal and GaN substrate discussed above have two types of domains that differ in polarity. The two domains are defined as the principal domain (matrix), which is the majority, polarity-determining domain of GaN crystal and GaN substrates, and inversion domains, which are domains in which the polarity in the [0001] direction is inverted with respect to the matrix. This means that on a (0001) Ga face that is the principal plane of a GaN substrate, both the (0001) Ga face of the matrix as well as (000 1) N faces of inversion domains appear. Therefore, when a (0001) Ga face that is the principal plane of a GaN crystal is etched, the inversion domains become more etched than the matrix, such that approximately hexagonal columnar pits form from the inversion domains. In other words, the hexagonal columnar pits are pits that originate in the inversion domains.
Meanwhile, along the principal plane of a GaN substrate, pits originating in dislocations are not the result of etching with KOH solution at 50° C. for some tens of minutes, but are the result of etching with the molten KOH/NaOH mixture. Yet since they are in the form of hexagonal pyramids having ridgelines, pits originating in dislocations are readily distinguished from pits originating in inversion domains. It should be noted that, other than by the etching mentioned above, the principal and inversion domains can be readily distinguished from each other also by cathodoluminescence (CL), or by observation under a florescence microscope, because the luminosities of the two domains differ distinctly.
In implementations in which GaN crystal is grown on a non-native substrate, a low-temperature buffer layer is generally formed on the non-native substrate, as is the case in Non-Patent Literatures 1 and 2, but in thus growing GaN crystal on a non-native substrate with a low-temperature buffer layer intervening, inversion domains inevitably form. This has meant that general GaN crystal will contain inversion domains.